1. Field of the Invention
The present invention is concerned with a method for controlling a stepping motor comprising a coil, a rotor coupled magnetically to the coil and means for bringing the rotor into, or maintaining it in, at least one given rest position in the absence of current in the coil.
The present invention is also concerned with a device for controlling such a stepping motor.
2. Description of the Prior Art
The electric energy necessary for driving the mechanical elements connected to a stepping motor is generally supplied by a control circuit which delivers a driving pulse every time the motor is to advance by one step. The driven elements may be the elements such as hands and/or discs displaying the time information given by an electronic timepiece.
A considerable reduction in the electric energy consumed by the motor can be obtained by providing in the control circuit a circuit which adjusts the energy of the driving pulses to the minimum corresponding to the actual mechanical load driven by the motor. There are various types of circuits for measuring this actual mechanical load and adjusting the energy of the driving pulses.
U.S. Pat. No. 4,212,156, for example, describes a control circuit in which the duration of each driving pulse is already determined before it begins. A detector circuit measures the time that elapses between the end of each driving pulse and the appearance of the first minimum of the current induced in the coil by the oscillations of the rotor about its position of equilibrium.
If this time is small, this indicates that the load driven by the rotor during this driving pulse was likewise small and therefore that the rotor has certainly finished its step. The control circuit does not modify the duration of the following driving pulses or, according to circumstances, reduces this duration. If, on the other hand, this time is long, this indicates that the load driven by the rotor was considerable and that the rotor has perhaps not turned in response to this driving pulse. The control circuit then sends a correction pulse of long duration and of the same polarity as the driving pulse which has just finished and increases the duration of the following driving pulse.
In such circuits, the detection of the rotation or non-rotation of the rotor is therefore effected immediately, or almost immediately, after each driving pulse. These circuits will be called immediate detection circuits in the following description.
U.S. Pat. No. 4,300,233 describes another kind of control circuit in which the duration of each driving pulse is predetermined. In this circuit, a detector circuit measures the intensity of the current flowing in the coil of the motor about two milliseconds after the beginning of each driving pulse. If this intensity is lower than a predetermined value, this indicates that the rotor is in the correct position for turning in response to this driving pulse and therefore that it has turned in response to the preceding driving pulse. If this intensity is higher than the predetermined value, this indicates that the rotor is not in the correct position and therefore that it has not turned in response to the preceding driving pulse. The control circuit then interrupts the driving pulse in progress, sends to the motor a correction pulse of the same polarity as the preceding driving pulse and then again sends the normal driving pulse. In such circuits, the detection of the rotation or non-rotation of the rotor in response to a driving pulse is therefore effected a long time after the end of this driving pulse. These circuits will be called delayed detection circuits in the following description.
It should be noted that, whatever the kind of control circuit and of adjustment used, the duration of the driving pulses is generally less than the time taken by the rotor to carry out its step. The electric energy supplied to the motor by each driving pulse is as a rule sufficient for the rotor to finish its step due to the kinetic energy which it has accumulated and to a positioning torque which tends to bring it back or maintain it, in the absence of current in the coil, into or in a rest position of stable and definite equilibrium. This positioning torque is created by a special shape given to the pole pieces which surround the rotor of the motor, or by one or more positioning magnets.
The curve 1 of FIG. 1 illustrates diagrammatically the variation in this positioning torque as a function of the angle of rotation a of the rotor between two rest positions corresponding to the points A and B. When this torque is positive, it tends to cause the rotor to turn in the direction of increase of the angle a and, when it is negative, it tends to cause it to turn in the direction of decrease of this angle a.
In the majority of motors used at present in timepieces the rotor turns in steps of 180 degrees, which means that it has two rest positions per revolution. In other types of motor, the step of the rotor corresponds to a rotation of 360 degrees, which means that the rotor has only one rest position.
The period of the positioning torque is equal to the angle between two successive rest positions of the rotor. There is therefore a position of the rotor, represented by the point C in FIG. 1, which corresponds approximately to a rotation of half a step, at which this torque is null and changes sign. The sense of the torque to either side of C is such as to drive the rotor away from C. This point C therefore corresponds to a position of unstable equilibrium of the rotor.
The mechanical load driven by the motor is constituted for a large part by the resisting torque due to the unavoidable friction of the pivots of the rotor and of the toothed wheels which it drives in their bearings, and also by the friction of the teeth of these wheels between them. This frictional torque is represented diagrammatically by the curves 2 and 3 in FIG. 1.
Around the point C of unstable equilibrium there is a zone, defined by the points D and E, in which the frictional torque is greater than the positioning torque. If the energy supplied to the rotor by a driving pulse is sufficient for it to reach and pass the point D, but is not sufficient for it to reach and pass the point E, the rotor then remains blocked in an intermediate position which may be located anywhere between these points D and E.
FIG. 2 illustrates diagrammatically a motor of the type most currently used in electronic timepieces in the situation where its rotor is blocked in such an intermediate position. FIG. 2 shows the coil 11, two pole pieces 12 and 13 which form part of the stator of the motor, and the magnet 14 of the rotor. The magnetization axis of this magnet 14 is represented by the arrow 15, which is directed from its south pole towards its north pole. In this example, the positioning torque of the rotor is created by notches 16 and 17 formed in the pole pieces 12 and 13, respectively.
In normal operation, the control circuit of the motor, not shown in FIG. 2, delivers driving pulses to the coil 11 in response to control pulses supplied, for example, by a time base circuit each time that the rotor is to advance by one step.
All the explanations which are to follow will be given taking such a motor as an example. However, the expert will appreciate that they apply without any difficulty to any type of stepping motor.
For these explanations, it will be assumed that the point A in FIG. 1 corresponds to the position of the rotor in which the magnetization axis of its magnet is represented by the dashed arrow 15' shown in FIG. 2 and that the rotor has been brought to the position represented by the arrow 15 by a driving pulse designated by the reference 18 in FIG. 3 and applied to the coil 11 so that the pole piece 12 acts as a south magnetic pole and the pole piece 13 acts as a north magnetic pole. The energy supplied to the motor by this pulse has been sufficient for the rotor to reach a position located beyond the point D in FIG. 1, but, for some reason, it has been insufficient for the rotor to go beyond the position corresponding to the point E. The rotor is therefore remained blocked in the intermediate position shown in FIG. 2.
If this situation occurs with an immediate detection control circuit of the kind of that which is described in U.S. Pat. No. 4,212,156 mentioned above, this control circuit sends a correction pulse to the motor as soon as it detects that the rotor has not finished its step. This correction pulse, which is designated by the reference 19 in FIG. 3, has the same polarity as the driving pulse 18 and a duration long enough for causing the rotor to turn by a complete step, from the point A to the point B. The torque created by this pulse is shown by curve 4 in FIG. 1. As, in this case, the rotor is in a position located between the points A and B, this correction pulse is not yet finished when the rotor reaches a point B' which is the point where the positioning torque and the torque created by the current in the coil cancel each other. The rotor oscillates about this point B' and, at the instant when the correction pulse ends, it is very possible that the rotor has a speed and a direction of rotation such that it starts off again in the direction of the point A and completes its step in the opposite direction.
This case is illustrated in FIG. 3, in which the references 18 and 19 designate the driving pulse which has brought the rotor into the position of FIG. 2 and the correction pulse, respectively, and in which the curve 20 represents diagrammatically the angular position of the rotor as a function of time.
In such a case, the correction pulse does not achieve its purpose, which is to make up for a preceding driving pulse whose energy has been insufficient to cause the rotor to turn correctly.
The same situation may arise if the rotor is not really stopped at the end of a driving pulse, but its rotation has simply been retarded for one reason or another. In this case likewise, the correction pulse sent by the control circuit produces oscillations of the rotor around the point B' and the rotor may very well be sent back to the point A at the end of this correction pulse.
In the case where the control circuit of the motor is of the kind of that which is described in U.S. Pat. No. 4,300,223 already mentioned, the detector circuit may not supply its detection signal if the rotor has been blocked in an intermediate position close to the position B. The driving pulse which follows that during which the rotor has been blocked is not then interrupted and the rotor returns to its starting position. If the position in which the rotor is blocked is such that the detector circuit reacts to this situation, the control circuit sends a correction pulse whose effect may be the same as in the cases above.
To sum up, it will be seen that if the rotor of the motor remains blocked in an intermediate position, the known control circuits comprising a circuit detecting the non-rotation of the rotor do not guarantee perfect operation of the motor in all cases.